The high electron mobility transistor (HEMT) is a type of field effect transistor (FET) in which a hetero-junction between a channel layer and a barrier layer whose electron affinity is smaller than that of the channel layer is formed. A two-dimensional electron gas (2DEG) forms in the channel layer of a group III-V HEMT device due to the mismatch in polarization field at the channel-barrier layer interface. The 2DEG has a high electron mobility that facilitates high-speed switching during device operation. In typical HEMT devices, a negatively-biased voltage may be applied to the gate electrode to deplete the 2DEG and thereby turn off the device. A group III-V HEMT device is one made of materials in column III of the periodic table, such as aluminum (Al), gallium (Ga), and indium (In), and materials in column V of the periodic table, such as nitrogen (N), phosphorus (P), and arsenic (As).
FIG. 1 shows a cross-sectional view of a prior art structure for an HEMT device. The HEMT device 100 shown in FIG. 1 begins with substrate 102, which can be silicon (Si), silicon carbide (SiC), sapphire (Al2O3), or any other suitable substrate for epitaxially growing layers of group III-V materials. For substrates other than bulk gallium nitride (GaN), it is difficult to epitaxially grow a high-quality gallium nitride (GaN) semiconductor crystal layer on substrate 102 due to poor lattice matching between the gallium nitride (GaN) and the substrate material. As such, an optional buffer layer 104, also known as a nucleation layer, can be deposited on substrate 102 to provide a surface on which high-quality gallium nitride (GaN) may be grown. Buffer layer 104 can be gallium nitride (GaN), aluminum gallium nitride (AlGaN), aluminum nitride (AlN), or any other suitable material for growing gallium nitride (GaN). Epitaxial growth of gallium nitride (GaN) forms a channel layer 106 on buffer layer 104. The channel layer 106 may be formed by any known process, including metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or any other suitable growth technique.
Next, a barrier layer 108, also known as an electron supply layer, may be formed by epitaxial growth on channel layer 106. Barrier layer 108 may be made of aluminum gallium nitride (AlxGa1-xN), indium aluminum nitride (InxAl1-xN), or any other material suitable to form a hetero-junction with the gallium nitride (GaN) based channel layer 106. Electrodes 112 and 114 formed on barrier layer 108 act as the source and drain, respectively, of the HEMT device 100. Source and drain electrodes 112 and 114 may be titanium (Ti)/silicon (Si)/nickel (Ni), titanium (Ti)/aluminum (Al)/nickel (Ni), or any other suitable material that forms an ohmic contact with the barrier layer 108. Gate electrode 110 is also formed on barrier layer 108, between the source electrode 112 and drain electrode 114. Gate electrode 110 comprises a material that forms a non-ohmic contact (a contact which does not exhibit linear I-V characteristics) with the barrier layer 108.
During device operation of the foregoing HEMT device 100, a 2DEG forms on the channel layer side of the interface between channel layer 106 and barrier layer 108, allowing current to flow between the source electrode 112 and the drain electrode 114. A negative voltage (relative to substrate 102) may be applied to gate electrode 110 to deplete the 2DEG and shut off the flow of current between the source electrode 112 and the drain electrode 114, turning off the HEMT device 100.
To improve the electrical breakdown performance of the HEMT device 100, carbon (C) can be incorporated into the gallium nitride (GaN) based channel layer 106 to increase the electrical resistivity of the gallium nitride (GaN) material. While carbon (C) is naturally present in small concentrations in the gallium nitride (GaN) based channel layer 106, greater quantities of carbon (C) can be introduced in the gallium nitride (GaN) material (also known as carbon doped gallium nitride (c-GaN)) by altering the growth conditions of the gallium nitride (GaN) channel layer 106. Specifically, this infusion of carbon (C) can be achieved by growing the gallium nitride (GaN) channel layer 106 at low temperature, high growth rate, and a low ratio of group-V precursors to group-III precursors. However, the growth conditions that promote the incorporation of carbon (C) in gallium nitride (GaN) are in direct conflict with the growth conditions necessary to grow high-quality gallium nitride (GaN), which include high temperature, low growth rate, and a high ratio of group-V precursors to group-III precursors.
Because carbon doped gallium nitride (c-GaN) has inferior crystal quality and morphology, manufacturers are unable to grow the carbon doped gallium nitride (c-GaN) in thick layers, limiting the electrical breakdown performance of the HEMT device 100. The structural defects present in carbon-doped gallium nitride (c-GaN) may also result in poor device performance and lower yield per wafer due to structural deterioration of the carbon-doped gallium nitride (c-GaN) material. Moreover, a thick layer of carbon-doped gallium nitride (c-GaN) makes the HEMT device 100 unsuitable for an increasing number of end applications; particularly in light of the growing demand for smaller FET devices.
There is, therefore, an unmet demand for thinner HEMT devices with improved electrical breakdown performance and improved structural quality.